Acid pre-treatment method for in situ ore leaching

ABSTRACT

An acid leaching method for the recovery of a desired element from a subterranean rubblized body of primary ore containing the element and also having associated therewith a carbonate mineral wherein the rubblized ore body is flooded with an aqueous acidic solution in order to release carbon dioxide from the associated carbonate mineral. After a substantial portion of the available carbon dioxide is released and removed from the ore body, as by venting to the atmosphere, an oxidizing gas is introduced into the flooded, rubblized ore to oxidize the ore and form an acid leach solution effective in the presence of the dissolved oxidizing gas to dissolve the ore and cause the desired element to go into solution. The leach solution is then circulated to the surface where the metal values are recovered therefrom.

United States Patent Mallon et al.

[45] Oct.28, 1975 [54] ACID PRE-TREATMENT METHOD FOR IN SITU ORELEACHING Primary Examiner-Ernest R. Purser [75] lnvemors Richard GManon. Robert L Attorney, Agent, or Firm-Dean E. Carlson; FrederickBraun, both of Livermore, Calif. A" Robertson Irene S' Croft [73]Assignee: The United States of America as [57] ABSTRACT represented bythe Umted States An acid leaching method for the recovery of a desiredEner gy Research and Development element from a subterranean. rubblizedbody of pri- Adm'mstrater'i Washmgtoni D-C' mary ore containing theelement and also having asso- [22] Filed: July 23, 1974 ciatedtherewitha carbonate mineral wherein' the rubblized ore body is floodedwith an aqueous acidic solu- [21] Appl' NO': 491,081 tion in order torelease carbon dioxide from the associated carbonate mineral. After asubstantial portion [52] U.s. ci ..299/4; 75/101 BE, 75/117 of theavailable eeiboii dioxide is released eiid re- [51] im. CL2 E213 43/28;Ezic 41/14 moved fioiii the .oie body, as by veiiiiiig to the otiiio-[58] Field of Search 299/4, 5; 75/101 R, Sphere, aii oxidizing gee iSintroduced iiito the flooded, 75/101 BE, 75/117; 166/247 rubblized oreto oxidize. the ore and form an acid i leach solution effective in thepresence of the dis- [56] References Cited solved oxidizing gas todissolve'the ore and cause the UNITED STATES PATENTS desired element togo into solution. The leach solution 2 302 136 H1942 M 299/45 D is thencirculated to the surface where the metal valll'lOn i i s are rec veredtheref o 3,506,069 4/1970 Natiand 16e/247 ue o r m 3,640,579 2/1972Lewis 299/4 8 Claims, 1 Drawing Figure SOLUTION PROCESSING PLANT -20oxiDlzlNG GAS SOURCE l 4; 1 i i Q( 7s-(l: 0.8 9 fie's Iii: l "i: 10 2)is; fhmgfo'lzf'a 9J. e. o fbx/r1', l Affi l Il" e gill a 'Q0 plee V51/,Hl( k/--' -t van, 04.? 0,0 galla/ f -`l {vf/Jil J, a? e" ''f' .Q e"Hi/1"' i'fe: dei J '35 Pf`y d ofovgff: {Mi/,2 Cita/545 1 (Ygr izl/urnas@ geg ne, 16 WHL nfl-|7915; 19 e, PJJ/M L p5* 1 1 ffll') L-'z'f"fa. .lief-'f1' i' fr 1 Aqua izzjgalgib .93- 9 .,L l.' .l 7l/f f7 CIl "tfa'h-d V|,li" .-,f 5:85 ,5 ,-51 SSIUHY i-Iowa pQ- uInJ .l .l/" l t,.Q-:Cfiit 12 fwn, i buv, ,wl), n i1-Pyr ,dgn/r .Lmkif ,.p'D-gu y, f',$f.( '.(Z/l//Atl ,gljghxd--rn' 'sd'g f {vl} -f 1; J`'l-gd iuefr'jf;fling lr' i. n.170' qw; 'C ,'siri {rsr i "2.1721: {eff/{lidi} 57,21 ,jfi 7;? l fj lctr," 5:" "r: :"\;`r" i/Lf I t r' l l liv/: .Lflr/,'f l@r11-Tf n L l 'i -r- ;i'rg, ve

SOLUTION PROCESSING PLANT UgS. Patent oct. 2s, 1975 GROUND SURFACE ACIDPRE-TREATMENT METHOD FOR IN SITU ORF. LEACHING The invention disclosedherein was made in the course of or under contract W-7405-ENG-48 withthe United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to the acid leachingof primary ores, specifically, primary sulfide ores having carbonateminerals associated therewith, for the recovery' of metal valuestherefrom.

Large ore deposits, particularly copper ore deposits, typically have arelatively shallow oxidized secondary ore zone above the water table anda much deeper unoxidized primary ore zone below the water table. Many ofsuch primary deposits have been upgraded by processes of oxidation andsupergene enrichment to concentrations that make them economicallyfeasible to mine. The enrichment process is a near-surface process. Mostore deposits have been developed by mining the enriched and oxidizedzone thereof. Only where the primary ore is rich enough has it beeneconomical to mine at depth or, with large and efficient earth-movingequipment, to mine large deposits in open-pit mines.

Leaching techniques have been applied economicallyV to the oxidizedparts of ore deposits or to those that can be oxidized by alternatesolution and weathering or bacterial oxidation. Because of the lowpermeability of such ore deposits, this has not been done in place, withthe exception of some leaching operations conducted on caved material inmine workings. Hence, large deposits of primary ores, e.g., primarysulfide ores, remain undeveloped because they lie too deep to mineeconomically, their size is too small to allow the economics oflarge-scale mining operations, the grade is too low, or variouscombinations of these factors. Reserves of copper and other elementswould be considerably increased if an economical process of obtainingsuch elements from deep primary ore deposits were available.

With a view towards economic recovery of elements from deep oredeposits, it has been proposed to break up deep ore deposits, e.g., bycontained nuclear explosion or by conventional mining methods such asblock caving, to form a rubblized ore body, and then percolate a leachsolution through the rubblized ore to oxidize the broken ore and causethe desired element to go into solution. Certain ores, such aschalcopyrite (CuFeS2) and most other primary sulfide minerals, requirethe addition of an oxidizing gas, such as oxygen, to the leach solutionor the ore in order to effectively leach the desired element therefrom.The oxygen, dis- Vsolved in water, oxidizes some of the sulfides,particularly pyrite (FeS2) and produces an acid leaching solution whichin the presence of the dissolved oxidizing gas dissolves the ore. Suchacid leaching processes are disclosed, for example, by Arthur E. Lewis,U.S. Pat No. 3,640,579, In Situ Pressure Leaching Method, issued Feb. 8,1972, and in the copending application of Arthur E. Lewis, Ser. No.347,757, filed Apr. 4, 1973 now U.S. Pat. No. 3,823,981 issued July 16,1974.

In certain areas, however, the primary ore deposits have associatedtherewith carbonate minerals in quantities sufficient to interfere withthe acid leaching process. The acidic leaching solution reacts with thecarbonates present to liberate carbon dioxide which suppresses theleaching reactions by interfering with the transport of oxygen to thedesired reaction sites. Furthermore, any undissolved oxidizing gasdischarged from the rubblized ore region is contaminated with carbondioxide which will further interfere with the transport of oxidizinggas.

SUMMARY OF THE INVENTION lt is, therefore, among the objects of thisinvention to provide an improved acid leaching process for the recoveryof valuable elements from their primary ores, more particularly, fromprimary sulfide deposits having carbonate minerals associated therewith.

In accordance with the process of the present invention, well knowntechnology, such as underground nuclear technology or conventionalmining technology, is employed to form a rubblized ore region or chimneyin a subterranean ore deposit, preferably well below the water table, toserve as an in situ leaching vessel. Thereafter, as a salient feature ofthe invention, the rubblized region is flooded with acidized water whichdecomposes at least a portion of any carbonate minerals present, therebyliberating carbon dioxide which is then discharged from the ore body,eg., by venting to the atmosphere. After a substantial portion oftheavailable carbon dioxide has been released and discharged from therubble region, an oxidizing gas, preferably oxygen, is introduced intothe region in order to form the oxygenated leach solution and dissolvethe ore, as described above. The leach solution is subsequently pumpedtothe surface and the desired element recovered therefrom byconventional processes, such as replacement, electrolysis, hydrogenreduction, ion exchange, solvent extraction, etc.

Generally, according to the present invention it is preferred to provideand utilize a nuclear chimney created by the detonation of a containednuclear explosive in a primary ore deposit well below the water table,as in the leaching operation disclosed in the above-cited Lewis U.S. PatNo. 3,640,579. The broken ore in the filled chimney is thus subjected tohydrostatic pressures which are dependent upon the prevailing waterhead. The hydrostatic pressure acting on the chimney is utilized toenhance the dissolution of oxygen, thereby increasing the leaching rate.

BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in theaccompanying drawing wherein the single FIGURE is a crosssection of ageological formation with a rubblized ore region formed in a primary oredeposit and adapted to the conduct of a leaching method in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing indetail, there is shown a primary ore deposit 10 which includes compoundsof a valuable element which is desired to be obtained. For example, inthe case of copper the ore may comprise copper sulfides such aschalcopyrite, bornite, Chalcocite, etc. Thereafter, a chimney 1l ofbroken ore is formed in the deposit. Although the column of rubblizedore is best prepared using a nuclear explosive placed well below thewater table as noted above, preparation is'also possible` with moreconventional mining processes, eg., block caving.

The leaching operation of the present invention is preferablyaccomplished by drilling at least one shaft l2 which is provided with acasing to communicate with the bottom of the chimney ll and at least oneborehole 13 to communicate with the top portion thereof. The lower endof the casing of shaft 12 may be perforated over a length extendingacross the bottom of the cavity so as to disperse injected materialsmore effectively. Borehole 13 is provided with pipes 14 and extendingtherethrough in spaced relation to each other and to the sides of theborehole, pipe 14 communicating with the top portion of the chimney forextraction of leach solution and pipe 15 communicating with the topportion of the chimney for discharge of gaseous material therefrom.Often times a void space 16 of considerable volume is formed in theupper portion of the chimney in which event pipe 15 communicates withthe void space. Sufficient acidized water is then introduced via casedshaft or well bore 12 to fill the rubble column or chimney until thenatural water level is restored. Since the rubblized ore column ispreferably well below the water table, column l1 can be flooded at leastpartially by the natural inflow of water under the hydrostatic head ofits vertical displacement below the water table. However, naturalflooding is relatively time consuming, and, consequently, it ispreferred to provide most of the required water by artificial inflow. Toprovide conditions suitable for the subsequent leaching operation, thepH of the flooding solution is preferably lower than that desired forleaching since a portion of the added acid will be consumed by reactionwith carbonate minerals. For example, in the case of recovery of copperfrom primary copper sulfide ores the pH of the flooding solution ispreferably below substantially 3, specifically in the range of fromsubstantially 0.1 to substantially 2.

The acidized water added to column 11 reacts with carbonate mineralspresent to liberate carbon dioxide which rises upward through thechimney and through pipe l5. The thus released carbon dioxide is thenvented to the atmosphere by means of line 17. Reaction with carbonatesis permitted to proceed until a substantial portion of the availablecarbon dioxide is released and discharged from the rubblized ore. Thisacid flooding and pretreatment stage may require about days or more,depending upon the proportion of carbonate mineral present in theprimary ore and the volume of rubble column.

After a substantial portion of the carbon dioxide has been dischargedfrom the rubblized ore, vent 17 is closed, and an oxidizing gas is thenintroduced to the bottom of the chimney via the shaft 12 as by means ofa pipe 18 extending at least partially therethrough in spaced relationto the shaft wall and connected at its upper end to a suitablecompressed gas source 19. Preferably, pipe 18 terminates in a perforatednozzle section somewhat above the top of the chimney. Thus, theoxidizing' gas exits as a stream of fine bubbles substantially uniformlydispersed in the downwardly flowing liquid leach solution; the bubblesof gas are carried downward .with the liquid stream and are thuscompressed by the increasing hydrostatic pressure. This method ofinjection also greatly reduces the problem of anhydrite (anhydrouscalcium sulfate) deposition; if anhydrite should be deposited in theinjection pipe, the pipe is readily accessible for maintenance.

The oxidizing gas is introduced into the bottom of chimney ll at apressure equal to or slightly above the hydrostatic pressure. As thebubbles rise through the chimney, part ofthe gas dissolves. Theundissolved gas, as it rises through the chimney, provides a liftingforce that induces enough circulation in the chimney to carry dissolvedgas to all parts of the chimney. Since a major portion of the carbondioxide is removed in the acid pre-treatment stage, only minor amountsof carbon dioxide are liberated during the leaching operation. Thus,circulation of the oxidizing gas and the subsequent oxidation reactionscan proceed without interference from substantial amounts of carbondioxide. By virtue of the high pressure in the chimney, the solubilityof the oxidizing gas is increased such that it readily dissolves andoxidizes the ore minerals to form an acid leach solution in thereaction. The acid solution aids the oxidation of the ore minerals bythe oxidizing gas and the desired ore element goes into solution at arelatively rapid rate. Any excess undissolved oxidizing gas rises to thesurface and is discharged via pipe l5 for reuse in the leachingoperation. As a Consequence of the acid pre-treatment step of thepresent invention, the discharged oxidizing gas will not be contaminatedwith substantial amounts of carbon dioxide and can therefore be feddirectly into resupply pipe 18.

Leach solution is extracted from the chimney, preferably by pumping sameto the ground surface as by means of a pump (not shown) communicatingwith the chimney via pipe 14. The pump delivers the pregnant leachsolution to a solution processing plant 20 to recover the desiredelement therefrom arid/or recirculates the solution to the chimneythrough the clearance space existing between the pipe 18 and the wall ofthe shaft 12. In this manner the leach solution can be recirculateduntil the concentration of the desired element dissolved therein is highenough for profitable recovery, e.g., in the range of from about l toabout 20 or more grams per liter. Preferably, during production, thepregnant solution from the chimney is circulated to the surfacecontinuously and reintroduced into the chimney along with addedoxidizing gas, as hereinabove described, with a portion of the pregnantsolution from the chimney being diverted to a solvent extraction plantwhere it is stripped of the desired element and returned to the chimney.

From the above description, it can be seen that many advantages accrueto the leaching operation by virtue of the acid pre-treatment step ofthe present invention. The pre-release of carbon dioxide renders thesubsequent leaching operation more rapid and more eicient sincesubstantial amounts of carbon dioxide are not present during leaching tointerfere with the transport of oxidizing gas to the reaction sites andto suppress the leaching reactions. As a consequence, lesser volumes offluids are required to be circulated resulting in reduced costs forcirculation pumps and piping. ln addition, since very little carbondioxide will be discharged during leaching, the excess oxidizing gasdischarged from the rubblized ore region can be recycled, therebyreducing gas requirements and cost.

The acid utilized in the acid pre-treatment step can be any of the acidswhich are known to decompose carbonates and release carbon dioxide. Thecommon, inorganic acids, such as nitric, hydrochloric, sulfuric and thelike, are most useful for this purpose. Since the fragmented ore regionwill probably be very extensive,

ple.

EXAMPLE Consider the case of a deep deposit having an ore grade of 0.5percent copper, bulking factor of 0.15, and rock density of 2.7 g/cc inan area where the water table is at 50 meters. The lower limit of coppercontent which may be treated can be considerably lower while highercontents can even more readily be treated. From well established nuclearexplosive technology, it can be shown that a nuclear explosive with ayield of 100 kt emplaced and detonated in the ore deposit at a burialdepth of 750 meters is productive of a cavity or nuclear chimney with aradius of about 41 meters and volume of 2.9 105 cubic meters. Assuming achimney-wall slope of 3 from the vertical, it can be calculated that thechimney has a height of 250 meters above the shot point and contains 1.6X l06 cubic meters of broken ore. With the chimney thus positioned andproportioned, the hydrostatic pressure acting thereon when filled variesfrom 7000 kilopascals 1000 psi) at the bottom of the chimney to 4500kilopascals 650 psi) at the top. The resulting chimney is then filledwith an aqueous sulfuric solution having a pH of about 1 until theoriginal water level is restored. During this acid pre treatment stagethe solution is not circulated and very little leaching, if any, occursat this time. Carbon dioxide is liberated from the carbonate mineralsassociated with the ore and is vented to the atmosphere. The floodingsolution is in contact with the rubblized ore for a period of about 30days prior to introduction of oxygen and a substantial portion of theavailable carbon dioxide is discharged before the leaching operation isinitiated. Oxygen at a pressure of about 7000 kilopascals 1000 psi) isintroduced into the chimney at a rate of 140 cubic meters per hour for atime sufficient to provide an average oxygen concentration throughoutthe chimney solution equal to 0.65 of the solubility limit at thetemperature and pressure which prevail at the top ofthe chimney andproduce an acid leach solution in the pH range of from 1.5 to 3.0,preferably 1.5 to 2.0, whereby copper ore is dissolved and copper goesinto solution. The pregnant leach solution is then circulated to thesurface and used to compress oxygen as it is reinjected. Part of theflow discharged to the surface is diverted to a conventional solventextraction plant where it is stripped of copper and returned to thechimney. This part of the flow is passed through a heat exchanger todrop the temperature from about 90C to approximately 50C prior tosolvent extraction. The copper concentration to the extraction plant isof the order of from 'about 5 to about 10 grams per liter.

It will be appreciated that the leaching method of the present inventionmay be likewise applied to the recovery of other elements from theirprimary ores in a substantially similar manner to that detailed forcopper. For primary ores of elements similar to copper, oxygen is asuitable oxidizing gas. For ores of various other elements, such asgold, chlorine may be a suitable oxidizing gas for employment in theleaching process. Moreover, although the oxidation process is exothermicand the addition of heat to the chimney is generally not required onceproduction has begun, it may be desirable in some cases to add heatinitially so to reach optimum production more quickly. To this end theoxidizing gas and/or the recirculated solution may be heated prior toits introduction to the chimney, or steam may be injected into thechimney.

Although the invention has been hereinbefore described and illustratedin the accompanying drawing with respect to specific steps of the methodthereof, it will be appreciated that various modifications and changesmay be made therein without departing from the true spirit and scope ofthe invention` and thus it is not intended to limit the invention exceptby the terms of the following claims.

What we claim is:

l. An acid leaching method for the recovery of a desired element from asubterranean rubblized ore body of a primary ore containing same andalso having associated therewith a carbonate mineral comprising thesteps of:

a. flooding the subterranean rubblized ore body with an aqueous acidicsolution, thereby decomposing at least a portion of the associatedcarbonate mineral and releasing carbon dioxide,

b. discharging a substantial portion of the released carbon dioxide fromthe ore body,

c. introducing an oxidizing gas into the flooded, rubblized ore bodythereby oxidizing said ore to form an acid leach solution effective inthe presence of the oxidizing gas to dissolve said ore and cause saiddesired element to go into solution, and

d. recovering said desired element from said solution.

2. A method according to claim l further defined by maintaining theacidic solution in contact with the rubblized ore body for a period oftime sufficient to release a substantial portion of the available carbondioxide and discharge same from the ore body prior to introduction ofthe oxidizing gas.

3. A method according to claim 1 further defined by the aqueous acidicsolution being an aqueous sulfuric acid solution.

4. A method according to claim 1 further defined by the primary orebeing a primary sulfide ore.

5. A method according to claim 4 further defined by said desired elementbeing copper, said ore being primary copper sulfides, and said oxidizinggas being oxygen.

6. A method according to claim 5 further defined by flooding therubblized ore body with an aqueous acidic solution having a pH in therange of from substantially 0.1 to substantially 2.

7. A method according to claim 1 further defined by locating a primaryore deposit containing the desired element existing beneath the vwatertable, and detonating a contained nuclear explosive in said ore depositto form a chimney of rubblized ore at a substantial depth beneath saidwater table subjected to the hydrostatic pressure head of the verticaldisplacement of said chimney therefrom.

8. A method according to claim 7 further defined by said desired elementbeing copper, said ore being primary copper sulfides, and said gas beingoxygen.

:s i i k i

1. AN ACID LEACHING METHOD FOR THE RECOVERY OF A DESIRED ELEMENT FROM ASUBTERRANEAN RUBBLIZED ORE BODY OF A PRIMARY ORE CONTAINING SAME ANDALSO HAVING ASSOCIATED THEREWITH A CARBONATE MINERAL COMPRISING THESTEPS OF: A. FLOODING SUBTERRANEAN RUBBELIZED ORE BODY WITH AN AQUEOUSACIDIC SOLUTION, THEREBY DECOMPOSING AT LEAST A PORTION OF THEASSOCIATED CARBONATE MINERAL AND RLEASING CARBON DIOXIDE, B. DISCHARGINGA SUBSTANTIAL PORTION OF THE RELEASES CARBON DIOXIDE FROM THE ORE BODY,C. INTRODUCING AN OXIDIZING GAS INTO THE FLOODED, RUBBLIZED ORE BODYTHEREBY OXIDIZING SAID ORE TO FORM AN ACID LEACH SOLUTION EFFECTIVE INTHE PRESENCE OF OXIDIZING GAS TO
 2. A method according to claim 1further defined by maintaining the acidic solution in contact with therubblized ore body for a period of time sufficient to release asubstantial portion of the available carbon dioxide and discharge samefrom the ore body prior to introduction of the oxidizing gas.
 3. Amethod according to claim 1 further defined by the aqueous acidicsolution being an aqueous sulfuric acid solution.
 4. A method accordingto claim 1 further defined by the primary ore being a primary sulfideore.
 5. A method according to claim 4 further defined by said desiredelement being copper, said ore being primary copper sulfides, and saidoxidizing gas being oxygen.
 6. A method according to claim 5 furtherdefined by flooding the rubblized ore body with an aqueous acidicsolution having a pH in the range of from substantially 0.1 tosubstantially
 2. 7. A method according to claim 1 further defined bylocating a primary ore deposit containing the desired element existingbeneath the water table, and detonating a contained nuclear explosive insaid ore deposit to form a chimney of rubblized ore at a substantialdepth beneath said water table subjected to the hydrostatic pressurehead of the vertical displacement of said chimney therefrom.
 8. A methodaccording to claim 7 further defined by said desired element beingcopper, said ore being primary copper sulfides, and said gas beingoxygen.